A Study Of Low Cycle And Thermomechanical Fatigue Behavior Of Z2CND18.12N Autenitic Stainless Steel

As one type of austenitic stainless steel, because of it's good mechanical property , decay resistance and thermal stability, Z2CND18.12N is widely used in the designation of nuclear power plant equipment. In this study, with the utilization of MTS810, a series of room temperature and high temperature isothermal fatigue test in different total strain control were performed on this material, and the influence of temperature and strain on the life cycle was evaluated. It appears that the cyclic stress response of the materials under both Low Cycle Fatigue and ThermomechanicalFatigue condition displayed an initial hardening regime followed by a saturation period and then cyclic softening till failure, and the cyclic hardening has a relation to strain amplitude. Fatigue design curves at room temperature and high temperature are established and compared with the design curves proposed in ASME Code III, it appears that the curve in room temperature is adjacent to ASME curve, while the curve in elevated temperature is lower than the ASME curve on the whole.Tests in mechanical strain control were performed to understand the TMF (thermomechanical fatigue) behavior of this material under two different temperature domains. Different strains and phase-angles were applied. The stress response appears the same as the stress response under the LCF condition, but the hardening regime is longer. When the cyclic temperature is higher, the asymmetry of the stress-strain hysteresis loop is more evident. For the sake of comparison, total strain controlled LCF tests at the peak temperatures of TMF cycling were also performed, and the fatigue life of these tests are shorter than that of the TMF when the temperature range is higher. When the peak temperatures of TMF cycling is higher enough, dynamic strain aging begins to take place. This effect is more evident when it comes to the 550oC isothermal fatigue test. TMF cycling leads to the development of significant amounts of mean stresses. Specially, when the temperature range is low, the change of the mean stress is more evident. It shows that the higher the temperature range, the shorter the cyclic life is.Both optical and scanning electron microscopes were used to examine failed specimens. An optical microscope observation of the samples shows that surface crack initiations and crack propagations are typically transgranular mode.Some life prediction models were employed to predict the LCF and TMF life of Z2CND18.12N, and the results indicate that the energy-based models provide good prediction for thermalmechanical fatigue life of the material.